This invention relates to an automatic control for a refrigeration system and more particularly to a control for regulating the location of the saturated vapor point in the evaporator of a refrigeration system.
Conventional refrigeration systems employ a motor driven compressor, an evaporator for absorbing heat from a load, an expansion device for controlling flow of refrigerant into the evaporator, and a condenser for discharging heat from the system. The flow control device may comprise either a fixed capillary or orifice or a controllable expansion valve which can be controlled to vary the flow of refrigerant. Thus liquid refrigerant is admitted into the evaporator so that the heat absorbed from the load will warm the liquid refrigerant and evaporate the refrigerant. If the saturation point of the refrigerant vapor is not controlled to be at or very close to the outlet of the evaporator, but is instead allowed to occur in the evaporator at some distance from the outlet, the refrigerant vapor exiting the evaporator will be superheated, i.e., the refrigerant will be heated above its vaporization temperature. The number of degrees by which the vapor is superheated above its vaporization temperature is defined as the "superheat", expressed in degrees.
For an efficient refrigeration system, it is desired that the evaporator coil be fully wetted, i.e., that the saturation point is very close to or at the outlet of the evaporator. By thus controlling the saturation point, optimum evaporator coil performance is achieved.
Refrigeration systems such as described above are conventionally used with air conditioning systems. Such air conditioning systems may be subject to variable conditions. For instance, the desired temperature of the space to be controlled may be selected to be higher or lower, the outdoor ambient temperature may vary, and thus the cooling load of the space to be controlled may vary depending upon variations of the building loads. Thus the loading of an air conditioning system can vary greatly.
Relatively simple air conditioning systems are generally designed with a fixed restriction orifice device or capillary tube for metering liquid refrigerant into the evaporator. These systems will operate at an optimum operating point at a limited set of operating conditions. If conditions change which cause higher liquid pressure and/or lower evaporator coil loading there will be some liquid spillover at the evaporator coil exit. In that case there is too much refrigerant charge in the system for the operating condition. This reduces the system capacity and increases energy usage by the compressor. At lower heat loads or at lower liquid refrigerant pressure more superheat is produced in the evaporator because of the lack of charge, thus under-utilizing the evaporator coil surfaces.
Prior art control systems have dealt with variations in loading by providing expansion valves for controlling the metering of liquid refrigerant into the evaporator. One such valve is an electric expansion valve which required a certain amount of superheat in the refrigeration system for control of the valve. Generally the control consisted of two temperature sensing elements one of which was connected to the outlet of the evaporator and one of which was connected to some intermediate point within the evaporator coil. The difference in temperature between the two points was a measure of the superheat, and this temperature difference was used as a control variable. Some control systems have been designed which used the pressure and temperature of the evaporator outlet as a measure of superheat and have used these parameters to control the electric expansion valve.
Thermostatic expansion valves have also been widely used, which sense superheat indirectly by using the pressure of a refrigerant charged temperature sensing bulb to compare to actual pressure as a pressure equivalent of superheat.
A problem with such prior art systems has been that control of the expansion valve is by nature very sensitive to changes in temperature. Since such controls depended upon the production of some superheat in order to exercise control, it is difficult to control below a minimum superheat. As superheat setting is lowered toward zero, the control becomes more sensitive thereby causing over-control of the expansion valve and causing the system to "hunt" for a stable operating point. These control systems therefore inherently resulted in some inefficiencies of the refrigeration systems.
It is therefore desired to provide a control for a refrigeration system which eliminates hunting under steady-state conditions and which further eliminates the need for superheat so that the system will operate with the saturated vapor point of the refrigerant located at or very close to the outlet of the evaporator. Furthermore, it is desired to provide such a system which is simple and relatively inexpensive to construct. Lastly, it is desired to provide a control wherein excess refrigerant charge in the system can be stored when the heat load on the system is relatively small and which provides sufficient refrigerant charge to the system under heavy loading conditions.